Liquid crystal displays (LCDs) have been used in not only TV sets with a big screen but also small display devices such as the monitor screen of a cellphone. TN (twisted nematic) mode LCDs, which would often be used in the past, achieved relatively narrow viewing angles, but LCDs of various other modes with wider viewing angles have recently been developed one after another. Examples of those wider viewing angle modes include IPS (in-plane switching) mode and VA (vertical alignment) mode. Among those wide viewing angle modes, the VA mode is adopted in a lot of LCDs because the VA mode would achieve a sufficiently high contrast ratio.
Known as a kind of VA mode LCD is an MVA (multi-domain vertical alignment) mode LCD in which multiple liquid crystal domains are defined within a single pixel region. In an MVA mode LCD, an alignment control structure is provided for at least one of the two substrates, which face each other with a vertical alignment liquid crystal layer interposed between them, so that the alignment control structure faces the liquid crystal layer. As the alignment control structure, a linear slit (opening) or a rib (projection) of an electrode may be used, thereby applying alignment controlling force to the liquid crystal layer from one or both sides thereof. In this manner, multiple (typically four) liquid crystal domains with multiple different alignment directions are defined, thereby attempting to improve the viewing angle characteristic.
Also known as another kind of VA mode LCD is a CPA (continuous pinwheel alignment) mode LCD. In a normal CPA mode LCD, its pixel electrodes have a highly symmetric shape and either an opening or a projection (which is sometimes called a “rivet”) is arranged on the surface of the counter substrate which faces the liquid crystal layer so as to be aligned with the center of a liquid crystal domain. When a voltage is applied, an oblique electric field is generated by the counter electrode and the highly symmetric pixel electrode and induces radially tilting alignments of liquid crystal molecules. Also, with a rivet provided, the alignment controlling force by the slope of the rivet stabilizes the tilting alignments of the liquid crystal molecules. As the liquid crystal molecules are radially aligned within a single pixel in this manner, the viewing angle characteristic can be improved.
It is known that the display quality achieved by a VA mode LCD when the viewer is located right in front of the screen (which will be referred to herein as “when viewed straight on”) is significantly different from the one achieved when the viewer is located obliquely with respect to the screen (which will be referred to herein as “when viewed obliquely”), which is a problem with the VA mode LCD. Particularly when a grayscale tone is displayed, if adjustments are made so as to optimize the display performance when viewed straight on, then the display performance (including the hue and the gamma characteristic) achieved when viewed obliquely will be quite different from the one achieved when viewed straight on. The optic axis direction of a liquid crystal molecule is the major axis direction of that molecule. When a grayscale tone is displayed, the optic axis direction of a liquid crystal molecule is somewhat tilted with respect to the principal surface of the substrate. And if the viewing angle (or viewing direction) is changed in such a state so as to view the screen obliquely and parallel to the optic axis direction of the liquid crystal molecules, the resultant display performance will be totally different from the one achieved when viewed straight on.
Specifically, when viewed obliquely, the displayed image will look more whitish as a whole than when viewed straight on, which is called a “whitening” phenomenon. For example, if a person's face is displayed, the viewer will find that person's facial expressions displayed quite natural when viewing right in front of the screen. However, when viewing obliquely, he or she will sense that person's face look unnaturally white overall. In that case, subtle tones of the person's skin color may be lost and an overall whitish face may be displayed instead.
To minimize such a whitening phenomenon, multiple (typically two) subpixels may be formed by splitting a single pixel electrode into multiple (typically two) subpixel electrodes and applying two different voltages to those subpixel electrodes. In an LCD with such a multi-pixel structure, the grayscale characteristic of each subpixel is controlled so as to prevent the display performance from deteriorating even when viewed obliquely from what is achieved when viewed straight on (see Patent Documents Nos. 1 to 3, for example).
Specifically, in the LCD disclosed in Patent Document No. 1, the two subpixel electrodes are connected to mutually different source lines by way of two different thin-film transistors and a source driver applies mutually different source signal voltages to respective pixels. In that case, since the two subpixel electrodes will have different voltages applied, those subpixels will have respectively different luminances. Consequently, the whitening phenomenon can be less perceptible.
On the other hand, in the LCD disclosed in Patent Document No. 2, two different thin-film transistors associated with the two subpixel electrodes are connected to mutually different gate lines, and a gate driver applies different gate signal voltages to respective pixels so that the thin-film transistors will have mutually different ON-state periods. In that case, since the two subpixel electrodes will have different voltages applied, those subpixels will have respectively different luminances. Consequently, the whitening phenomenon can also be less perceptible.
With the LCD of Patent Document No. 1 adopted, however, the source driver should apply mutually different source signal voltages from two source output terminals to a single column of pixels. That is why a rather expensive source driver should be used in that case. The same goes for the LCD of Patent Document No. 2. That is to say, even if the LCD of Patent Document No. 2 is adopted, the gate driver should apply mutually different gate signal voltages from two gate output terminals to a single row of pixels, and therefore, a rather expensive gate driver should be used.
Meanwhile, Patent Document No. 3 discloses an LCD in which by making the voltages applied to adjacent storage capacitor lines (CS lines) vary to be different from each other, mutually different effective voltages are applied to subpixels. In the LCD of Patent Document No. 3, the source driver applies a source signal voltage from a single source output terminal to each column of pixels, and the gate driver applies a gate signal voltage from a single gate output terminal to each row of pixels. That is why the LCD of Patent Document No. 3 can cut down the driver cost.
Hereinafter, the configuration of the LCD 900 disclosed in Patent Document No. 3 will be described with reference to FIG. 10. In the LCD 900, by applying mutually different CS voltages to multiple CS lines, each of which forms, along with its associated subpixel electrode, a storage capacitor either directly or indirectly, the subpixel electrodes will have different effective voltages applied. As a result, subpixels belonging to a single pixel can exhibit multiple different luminances. The LCD 900 makes the whitening phenomenon less perceptible in this manner.
Also, in the LCD 900, a number of CS trunk lines CST are arranged in the peripheral area, which surrounds the display area, and multiple CS lines extend from each of those CS trunk lines to the display area. In the LCD 900, by applying equivalent storage capacitor voltages (CS voltages) to multiple CS lines that extend from the same CS trunk line, the processing load imposed on a CS voltage generator (not shown) can be reduced.
For example, the LCD 900 shown in FIG. 10 is provided with twelve CS trunk lines CST1 through CST12, and multiple different CS voltages, generated by the CS voltage generator, are supplied through those CS trunk lines CST1 through CST12 to the CS lines. In such an LCD, the larger the number of CS trunk lines provided, the longer the inversion period of the CS voltage applied to each of those CS trunk lines. If the CS voltage had an ideal rectangular waveform, then the display operation could be carried out with only two CS trunk lines without making the luminances uneven. Even so, the bigger the size of the LCD, the blunter the waveform of the CS voltage will get, thus making it impossible to carry out a display operation without making the luminances uneven after all. That is why by increasing the number of CS trunk lines to provide, the ratio of the CS voltage inversion period to one horizontal scanning period can be increased. As a result, the display operation can be carried out with the unevenness in luminance reduced significantly.